This invention relates to die cast zinc-base alloys and, more particularly, to a die cast zinc-aluminum-copper alloy exhibiting superior creep resistance.
In a typical die casting operation, molten metal is injected at high pressure into a fixed-volume cavity defined by reusable water-cooled metal dies. Within the cavity, the metal is molded into a desired configuration and solidified to form a product casting. The metal is injected into the cavity by a shot apparatus comprising a sleeve for receiving a charge of the molten metal and a plunger that advances within the sleeve to force the metal into the cavity. Two types of shot apparatus are known. A hot chamber apparatus comprises a shot sleeve immersed in a bath of the molten metal. In a cold chamber apparatus, the molten charge is transferred, for example by ladle, into the shot apparatus from a remote holding furnace.
Zinc-base alloys are commonly formed by die casting, in large part because of a conveniently low melting point. Heretofore, zinc die castings have exhibited a microstructure characterized by soft phases, such as the eta phase in zinc-aluminum alloys, that lack stability even at moderately high temperatures. As a result, such alloys have had poor high temperature creep resistance that has restricted their use, mainly to decorative parts.
It is known to form sand castings of a zinc alloy comprising between about 3 and 6 weight percent aluminum and between about 5 and 11 weight percent copper. Magnesium in an amount up to about 0.05 weight percent is also typically present. The alloy is characterized by a microstructure that includes a hard epsilon phase that increases bulk hardness and wear resistance in comparison to other common sand-cast zinc alloys. Thus, the alloy is well suited for material-forming dies such as sheet metal forming dies or plastic molding dies.
It has now been found that zinc-aluminum-copper alloys that form the epsilon phase not only are suitable for die casting by either cold chamber processes or hot chamber processes, but also exhibit a surprising increased creep resistance at elevated temperatures, which, in combination with their high hardness and wear resistance, allow the die cast alloy to be used in applications not suitable for conventional epsilon-free zinc-base die castings.
It is an object of this invention to provide a die casting composed of a zinc-base aluminum-copper alloy having a microstructure comprising a dispersed epsilon phase, which die casting exhibits not only high hardness and wear resistance, but also a remarkably increased high-temperature creep resistance, particularly in comparison to conventional zinc die castings.